There are many different iron oxide minerals, each with unique properties. Many of these minerals have properties that are useful for treating water contaminated with certain types of pollutants. Specifically, the minerals often have high specific surface area (area per gram of Fe) and surfaces that have a chemical attraction for a number of contaminants, including natural organic matter and many metals. As a result, contaminants that are initially dissolved in the water may adsorb onto (i.e., bind to the surface of) the minerals. The adsorption reactions tend to be quite pH-dependent, so that contaminants that are adsorbed at one pH can often be released when the solution is adjusted to a different pH. In general, anions (negatively-charged contaminants) are bound at lower pH values and released at higher pH values, while cations behave oppositely. This pH dependency is an important characteristic of the contaminant-surface interaction, because it means that, when the surface is saturated, the adsorbed contaminant can be easily desorbed so that the adsorption capacity can be reused to treat subsequent volumes of water. The exact pH range where contaminants are adsorbed or desorbed depends on the identity of the ion itself and the identity of the adsorbing surface. Thus, a generic statement that, e.g., `chromate is adsorbed at pH xx and desorbed at pH yy,` or that `iron oxide adsorbs cations at pH zz and releases them at pH ww` cannot be made. What can be said with some confidence is that under reasonable conditions, many iron oxides do adsorb many ionizable contaminants.
Even if contaminants of interest are adsorbed onto the particles, they still pose a potential hazard to the consumer of the water or to the environment unless the contaminant-laden particles are removed from the water. Unfortunately, the bulk properties of most iron oxides are not conducive to easy removal from the water. The particles that form when ferric ions are initially precipitated as ferric hydroxide solids tend to be small and highly hydrated, so their densities are only slightly greater than that of water. Accordingly, they settle very slowly. When packed in a small volume, they are virtually impermeable to water, so they cannot be used in a packed bed system (e.g., holding them in a confined space and passing water through them). For the same reason, it is very difficult to filter large quantities of these particles out of water, since the filter rapidly becomes clogged. Although the degree of hydration can be reduced, and the hydraulic conductivity can be increased by heating the particles, this significantly reduces the surface area available for adsorption and still yields small, difficult-to-handle particles.
Other adsorbents commonly used in water treatment include activated carbon and synthetic resins. Both of these are available in forms which can be packed in columns, so the water can be passed through the media without any need for subsequent solid/liquid separation steps. However, granular activated carbon cannot be regenerated on-site, and synthetic resins are often fouled by particulate matter.
In addition to dissolved contaminants, water also contains many contaminants that are not dissolved, but rather are in the form of suspended particles. These particles can be removed by some type of filtration process. Filtration through a bed packed with granular media (e.g., sand) is perhaps the most common technology used for this purpose. Desirable properties of media used for this purpose include high hydraulic conductivity, resistance to shear and abrasion (since the media are usually cleaned by fluidizing the packed bed), and negligible solubility under the conditions of use. Most filtration media do not have surface properties that are useful with respect to collecting dissolved contaminants from solution. Thus, there does not exist a convenient adsorbent/filter that is widely applicable for removing both soluble and particulate contaminants from water.
The potential advantages of having iron oxide (or other regenerable oxide adsorbents) available in a granular form have been recognized for many years, and a number of patents have been granted for processes to prepare such media. Several of these patents are discussed below. However, numerous forms of iron oxide exist, and neither their properties nor the transformations among them are well understood (except for transformations among a few, very pure forms). Furthermore, the factors that control the bond strength between an iron oxide surface layer and an underlying substrate are very unpredictable. As a result, while attempts to formulate such media might be guided by a rational set of hypotheses, the outcome is always uncertain.
U.S. Pat. No. 3,876,451 to Zall et al. describes a procedure for embedding a metal oxide in a matrix of activated carbon, for the purpose of removing phosphate anions from the water by the formation of an insoluble product containing the phosphate. In accordance with Zall et al., carbon is initially saturated with the metal by exposure to a solution of the metal chloride. Zall et al. notes that this can be achieved by exposing the carbon to the solution and then decanting the liquid, since the metal will be retained by the surface and in the pores of the activated carbon. After saturation, the material is dried and exposed to a solution containing an amount of sodium hydroxide in excess of the stoichiometric requirement. The material is then dried again and is ready for use.
U.S. Pat. No. 2,367,496 to Greentree describes the formation of an enhanced decolorizing agent which is made by incorporating iron into a hydrated magnesium silicate matrix. The first step described by Greentree is the exposure of magnesium silicate particles to an iron-containing solution, such that all the iron is adsorbed by the magnesium silicate. The direct use of this Fe-impregnated material, or the conversion of the impregnated Fe to Fe(OH).sub.3 by addition of a base, and, optionally, the heating of the Fe(OH).sub.3 -impregnated material to dryness, is described by Greentree.
U.S. Pat. No. 3,499,837 to Jaunarajs describes a process in which iron-oxide-coated media is used to collect phosphate from waste waters. The method for preparing the coated media involves exposure of a finely divided solid, such as diatomaceous silica filter aids, to a ferric sulfate solution, addition of a base to that solution to achieve near-neutral pH (e.g., pH 6.0 and 6.5), and drying of the resultant solid, either in air or at elevated temperatures.
U.S. Pat. No. 4,363,749 to Weiss et al. describes two methods for preparing adsorbent surfaces. The first involves "activating" the surface of an existing material. The second involves adding a solution containing iron (or other multivalent ion) to a suspension of the support material, and neutralizing with `an alkaline material.` In the examples of Weiss et al., the neutralization is to pH&gt;7. Weiss et al. emphasizes the need to use very small particles (diameter&lt;10 mm) and to avoid drying the material once the coating has been formed.
U.S. Pat. No. 4,459,370 to van der Waal describes that an iron-containing solution must be added very slowly to a suspension of the support material, and that the base necessary to neutralize and precipitate the iron onto the surface of the support is generated by the hydrolysis of urea or cyanate, which are added to the solution. The reaction takes place at pH 4 to 7 and at elevated temperatures.
A list of the properties that would be displayed by an ideal media might include:
Media must adsorb contaminants PA1 Media must be easily regenerable, in situ PA1 Media must resist abrasion associated with bed fluidization PA1 Media must resist dissolution when exposed to chemical conditions for adsorption and regeneration.
While pure iron oxides are known that meet the first two criteria, and several minerals are known that meet the last two criteria, there is a need to find a way to combine all four into a single media.